Metal-working



United States Patent 'O 2,7 92,627 METAL-WORIGNG Earle S. Thall, East Orange, N. .L, assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application March 9, 1954, Serial No. 415,177

12 Claims. (Cl. 29-556) This invention relates to an improved process of working and forming low ductility or malleability metals, particularly metals of the chromium group, i. e., chromium, tungsten, molybdenum or uranium or their alloys, in'order to impart to them a higherdegree of mall'eability and ductility and surface smoothness.

In a typical process of Wire making, powdered chromium, tungsten or molybdenum or an alloy including predominant amounts of any or all of these metals is formed under pressure into briquets which are swaged or hammered into rods. These rods are then coated with a graphite base lubricant and passed through successive dies at an elevated temperature to form wires of desired diameter. It is characteristic of this group of metals that they are kept below an annealing temperature to prevent brittle properties from occurring. Just prior to the drawing operation and in order to introduce the swaged coated rod into a die, an operation known as pointing is commonly practiced according to the method of Whitney, U. S. Patent 1,008,762. The pointing is accomplished by dipping the end of the wire in a bath of fused sodium nitrite or a similar oxidizing salt. The metal is removed at the tip of the rod to form a point. In the course of different stages of the drawing operation, it has been found that the removal of encrusted graphite and scale renders the material more workable according to the methods of Rich, U. S. Patent No. 1,663,564 and of Laise, U. S. Patent No. 1,470,175.

Even after such metals have been cleaned in the conventional ways described by Rich or Laise, they have brittle characteristics, with a comparatively low order of ductility and malleability, as compared with steel. Such brittleness and poor plasticity greatly inhibit drawing and working into filaments or wires.

Accordingly, it is an object of the present invention to increase the effective ductility of normally brittle metals such as those of the chromium group, i. e., chromium, tungsten, molybdenum, and uranium and alloys containing more than 20% of these metals such as Dowmo.

Another object of the present invention is to render more malleable metal parts composed of the chromium group, i. e., chromium, tungsten, molybdenum, and uranium and alloys containing more than 20% of these metals such as Dowmo.

Another object of the present invention is to increase the smoothness of the surface texture of metals of the chromium group, i. e., chromium, tungsten, molybdenum, and uranium and alloys containing more than 20% of these metals such as Dowmo in order that they may be more easily worked.

The advantages of the present invention are realized by removing from a cleaned specimen of metal an outer metallic skin of stressed metal which apparently is responsible for much of the inelastic brittle character of the ordinary cleaned metal.

The present invention is particularly applicable to the working of metals selected from the chromium group because these metals, unlike such metals as iron, become Patented May 21, 1957 less plastic, rather than more plastic when subjected to temperatures high enough to cause annealing. However, whenever an annealing operation is to be avoided, the present invention may be applied to sundry other worked, surface-hardened metals to cause increased ductility, malleability and surface smoothness.

The depth of the highly stressed skin of worked metal will, of course, depend upon the scope and nature of the prior operations performed upon the metal rods or wires. There is usually no sharp demarcation between the stressed layer and the unworked core but removal of 2 mils of metal from a 70 mil diameter swaged rod provides satisfactory ductility. This worked skin of metal does not seem to differ from the unworked inside core of metal in optical appearance. even under microscopic examination. However, X-ray diffraction pictures show a very marked difference in grain structure even though the boundary line between core and'skin may not be precise. The work-stressed outer layer is characterized by a highly fragmented grain structure while the unworked metal shows a substantially striated grain pat tern. By examination of X-ray diffraction pictures made of treated and untreated samples, it is readily apparent when the worked layer has been substantially removed from the treated material. The optimum degree of treatment can thus be observed by close attention to changes in the X-ray diffraction pattern. In general, the outer layer should be removed down to where the metal exhibits a predominantly striated grain structure.

Example A preferred example of a method in accordance with the present invention of making a tungsten or tungstenmolybdenum (Dowmo) wire of small diameter and having improved malleability and ductility is as follows: A block formed from pressed sintered tungsten powder is swaged or hammered at a temperature of about 1250 C. into a rod of about 70 mils diameter. The stressed, worked outer skin is removed by immersing the tungsten rod in a bath of fused sodium nitrate which is at a temperature of about 625 C. for about 10 seconds, to remove about 2 mils of the surface metal. The rod appears to be much more ductile after this operation and may then be drawn easily through a wire making die. As the bath of fused salt is contaminated through use, with by-prodnets of the process such as metal oxides, increasingly longer exposure becomes necessary to effect satisfactory etching. When fresh fused sodium nitrate at a temperature between 550 C. and 700 C. is used, a satisfactory etching occurs in less than 30 seconds on a 70 mil diameter specimen.

Etching can also be carried out electrolytically by making a tungsten rod the anode in an aqueous bath of 20% by weight caustic soda.

Other methods, both mechanical and chemical, may be used to remove the outer skin of stressed metal. The metal may be ground with abrasive powder. The outer stressed surface may also be removed by different types of etching agents. Sodium nitrite is an example of an oxidizing salt which may be used. Various mineral acids and alkalis will also remove surface metal.

Etching treatments can be alternated with successive drawing operations in the wire drawing process or even introduced between any or all of the swaging operations.

There have thus been described methods of improving the malleability, ductility and surface smoothness of certain metals, for example of the chromium group and alloys containing appreciable amounts of one or more of these metals, as well as other metals which are characterized by a work-hardened surface and a more workable core. This is accomplished by removing the work-stressed surface metal by etching, abrasion, or by electrolytic or othermeans.

What is claimed is:

1. A method of working a metal selected from the class consisting of chromium, tungsten, molybdenum, uranium and alloys containing more than 20% of said metalswhich comprises in combination the steps of swaging a body of said metal, drawing the swaged metal body to reduce its cross-sectional area, and further reducing said. area of said metal body by removing substantially all of the work- 'stressed metallic surface thereof at a temperature below the annealing temperature of said metal.

2. A method according to claim 1 wherein said removal' of the surface metal is accomplished by etching with a mineral acid. I

3. A method according to claim 1 wherein said removal of the surface metal is accomplished by-etching with a mineral alkali.

4. A method according to claim 1 wherein said removal of the surface metal is accomplished by drawing said metalthroughfused oxidizing salts of the alkali metals.

5. A method according to claim 1 wherein said removal of the surface metal is accomplished by drawing said metal through fused oxidizingsalts of the alkaline earths.

6. A method according to claim 1 wherein said removal of the surface metal is accomplished by etching electrolytically in a caustic bath. 7 a

7. "A method according to claim 1 wherein said removal of the surface metal is accomplished by mechanical abra- 8. A method of working a metal selected from the group consisting of tungsten, molybdenum, chromium, uranium and alloys containing substantial proportions of any of said metals which comprises in combination the steps of swaging a body of said metal, coating said swaged body with a lubricant, drawing the swaged material to reduce its cross-sectional area, removing adhering lubricants, also removing crust and scale which form on the surface of said body during the swaging and drawing operations, and removing substantially all of the workstressed metallic surface at a temperature below the annealing temperature of said metal.

9. In a process of working materials of the group including chromium, molybdenum, tungsten and uranium, and alloys thereof in order to render them more ductile, the step of removing the work stressed outer metallic skin formed during the Working operation. r j

10. A method of working a metal having a workstresscd surface which becomes less plastic at an annealing temperature which includes the steps of removing said surface at a temperature below the annealing temperature of said metal, and working the remaining portion of said metal.

11. In the process of working tungsten in order to render it more ductile, the step of removing the work stressed outer metallic skin formed during the working operation.

12. In the process of working alloys containing more than 20% molybdenum in order to render them more ductile, the step of removing the work stressed outer metallic skin formed during the working operation.

References Cited in the file of this patent UNITED STATES PATENTS 1,008,762 Whitney Nov. 14, 1911 1,285,057 Cutter Nov. 19, 1918 2,372,599 Nachtman Mar. 27, 1945 2,400,866 Kronwall May 21, 1946 2,478,856 Beaver Aug. 9, 1949 2,566,615 Keilholtz Sept. 4, 1951 2,570,748 Bain Oct. 9,1951 2,736,957 Heering Mar. 6,1956 

1. A METHOD OF WORKING A METAL SELECTED FROM THE CLASS CONSISTING OF CHROMIUM, TUNGSTEN, MOLYBDENUM, URANIUM AND ALLOYS CONTAINING MORE THAN 20% OF SAID METALS WHICH COMPRISES IN COMBINATION THE STEPS OF SWAGING A BODY OF SAID METAL, DRAWING THE SWAGED METAL BODY TO REDUCE ITS CROSS-SECTIONAL AREA, AND FURTHER REDUCING SAID AREA OF SAID METAL BODY BY REMOVING SUBSTANTIALLY ALL OF THE WORKSTRESSED METALLIC SURFACE THEREOF AT A TEMPERATURE BELOW THE ANNEALING TEMPERATURE OF SAID METAL. 